Well, that would depend on how you define a "simple" OS...
Anyway, it's not impossible, but incredibly difficult. And, given you know c/c++ and assembly you will also need a great deal of knowledge about processors and digital systems.
Linus Torvalds wrote the original Linux kernel by himself so it's obviously not impossible.
http://en.wikipedia.org/wiki/Linux_kernel

No prob man ^^
Yes, knowledge of C/c++ are pretty much compulsory (especially pointers and memory management) and as I mentioned earlier it would not hurt to learn (at least) the basics of assembly language as well.
You'll find tons of great guides on how and were to start by simply googling "how to write an os".
I actually tried to code a simple OS myself last year, but ended up barely starting because of lack of time. Anyway, I received a tip from some kernel programmers, "Read and modify code of some early OSes. Try implementing at least one of them on a modern computer and you'll learn a lot more than reading a tutorial".
Regardless of the method you chose it'll be a great challenge, but it's definitely not impossible.
Out of curiosity, why an OS?